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Do you mean the red door in the room you can teleport to after you do the Soulless One's quest? There's a keyhole on a wall in the east. You need Lord Infestor's key to activate that keyhole which will then allow you to tap on and open the door in the east.

To get to Lord Infestor's key, you need to open the gate in the south of that room. To do that, you Strength of 100. You can enchant some items if necessary.

That's Wizard Crotchety's Castle. If you bring him the Sugared Almonds, he thanks you and says you can use his machine beyond that door to travel to the Fourth Dimension. But, he mentions he lost the key. As you enter his castle, check the blue candle on the right.